The invention is directed to a process for electroplating metallic or non-metallic continuous products with metals or alloys in a continuous process from aprotic electrolytes free of water and oxygen. The invention is also directed to a device for performing said process.
According to the state of the art, continuous products such as wire, tapes, long-profiles, or pipes have been produced using aqueous electrolytic methods or by means of molten bath coating in a continuous process.
In a well-known electroplating process, for example, a wire is coated with various coatings, such as zinc, nickel or other metals where the wire is passed through open cleaning and electroplating baths containing aqueous solutions. In these baths, the respective metal is deposited on the wire, and the thickness of the coating layer depends on the passage rate and the electric field strength. In this process, however, the deposition rate as a function of time is rather low, and the deposited coating frequently is highly porous and rigid, giving rise to inferior corrosion resistance, particularly in thin coatings. As a result of lacking ductility, the subsequent forming procedures may give rise to cracks in the deposited layer or even flaking of the coating. Such a coating completely loses its corrosion-protective character and also, the surface is no longer decorative.
Furthermore, in the electrolytic deposition of a coating metal from an aqueous solution, the full-scale cathode or anode efficiency has never been reached. In general, side reactions occur at high current densities required for continuous coating, giving rise to decomposition products in the electrolyte and evolution of gas. Here, in particular, evolution of hydrogen takes place on the product, which may result in embrittlement of the basic material.
Another drawback is that the aqueous electroplating processes and hot-dip processes produce large amounts of toxic exhaust air and waste waters which must be purified by correspondingly expensive procedures where in any case, toxic special waste remains. For example, due to fat residues on the metals to be coated, which are present prior to alkaline cleaning in the appropriate solutions, residues of organic compounds are formed which, as a result of the high temperatures in the zinc tank, being around 450xc2x0 C., may react to give extremely toxic organic compounds such as dioxins and furans. Furthermore, metal sludges, used acids and exhausted alkaline cleaners are produced. In addition to the aforementioned waste gases, acid vapors and alkaline vapors are also produced.
Other processes for coating continuous products are known, which are based on the deposition of decorative and corrosion-reducing coatings in a molten state. In this context, the so-called zinc dipping (hot-dip galvanizing) and hot-dip aluminizing are known. In hot-dip galvanizing, a previously cleaned and activated continuous product, e.g., a thin wire is passed through high-purity molten zinc in a continuous process. This process takes place at temperatures above 440xc2x0 C., so that in any event, there is also a mechanical impact on the material to be coated. Due to the high temperatures, certain other basic materials desired to be coated cannot be coated at all. Another drawback is the relative non-uniformity of the deposited coating and the highly layer-dependent corrosion resistance. As a result of the stripping process, the surface may be void of any decorative character. A colored design of the surface is not possible.
In all the coating processes associated with zinc, there is blooming on the surface as a result of the formation of zinc oxides and zinc carbonates after a short period of corrosion and thus, a negative change of the surface with respect to the optical impression. Therefore, uniformity of the coating in these thermal processes is not ensured.
Another familiar process is the so-called high-temperature hot-dip aluminizing. In this process, a wire is drawn through a molten aluminum bath in the same way as in zinc coating and subsequently subjected to a stripping process. In this way, however, coatings are obtained exhibiting similar drawbacks as in hot-dip galvanizing described above. Layers coated by means of hot-dip aluminizing have not proven successful due to their insufficient purity, high porosity and inevitable oxidic inclusions and thus, inferior corrosion resistance. Other drawbacks can be seen in that the coating does not look decorative and in some cases, at the temperatures required for hot-dip aluminizing, there is a massive mechanical impact on the material to be coated.
In accordance with the state of the art, galvanizing may also be combined with hot-dip aluminizing, which results in a somewhat improved corrosion layer due to the active cathodic protective effect of the aluminum. However, the lacking decorative character is disadvantageous. In addition, there are other drawbacks for merely that reason that coating is effected at high temperatures.
The state of the art also includes aluminum electrodeposition processes performed in aprotic electrolytes free of water and oxygen wherein deposition of the aluminum is effected from baths containing alkyl aluminum complexes of alkali metal halides and aluminum alkyls. In general, aromatic or aliphatic hydrocarbons are used as solvents. Such electrolyte solutions are described in EP 0,402,761 A and EP 0,084,816 A, for example.
However, such electrolyte solutions have solely been used in the coating of rack products where the individual parts are arranged in appropriate racks to be immersed into the respective electrolyte baths. To date, aluminizing of continuous products using aprotic electrolytes free of water and oxygen is not known from prior art. Up to now, continuous products such as wires, tapes, long-profiles, and pipes are provided with a corrosion-inhibiting coating either by using electrolytic zinc plating in aqueous systems or hot-dip aluminizing or hot-dip galvanizing.
It is the technical object of the invention to provide a process which avoids the above-mentioned drawbacks of previously known coating processes for continuous products, is favorable in cost, and results in a superior coating. In addition, performing said process should be possible with no changes occurring in the basic material and particularly, at low temperatures.
The technical object is achieved by a process for electroplating metallic or non-metallic continuous products with metals or alloys in a continuous process from aprotic electrolytes free of water and oxygen, wherein the continuous product is passed through a lock system into an encapsulated coating plant under inert gas atmosphere, and the following steps are performed at temperatures of xe2x89xa6120xc2x0 C.:
activating the continuous product to be coated;
rinsing the continuous product to be coated;
contacting the continuous product to be coated;
electroplating the continuous product to be coated using a metal or metal alloy;
drying the coated continuous product;
discharge of the coated continuous product from the plant through a lock system.
In the meaning of the invention, continuous products are understood to be metallic or non-metallic materials which are produced in rolled or folded form and passed continuously through the plant in a continuous process during coating. Amongst these products are, e.g., wires of any thickness, tapes and long-profiles, pipes and similar products.
In the meaning of the invention, non-aqueous systems are designated as electrolytes, which permit controlled pure deposition of the metal or metal alloy, particularly aluminum and aluminum alloys by means of the electrolytic process, with no intermediate or support layer.
In a preferred embodiment, wire, tapes, long-profiles or pipes made of metallic or non-metallic materials are employed as continuous products. It is preferred that these materials be coated with aluminum or aluminum alloys.